The present invention relates to a disk player for reproducing, by means of an optical pickup, information from a disk such as a video disk, a compact disk, or the like, carrying an audio signal and a video signal recorded thereon.
The optical pickups used in optical disk players of the type as described above include an MM-type (moving magnet type) optical pickup, shown in FIG. 6, and an MC-type (moving coil type) optical pickup, shown in FIG. 7. The MC-type optical pickup presently is used more often.
In the MC-type optical pickup as shown in FIG. 7, coils 4 are attached to a movable portion 2 of an actuator having an objective lens 1. The movable portion 2 is driven in the focusing direction, in the tracking direction, etc., by forces generated between the coils 4 and magnets 3 provided outside the coils 4.
In the MM-type optical pickup as shown in FIG. 6, on the other hand, magnets 3 are attached to an outer periphery of a movable portion 2 of an actuator having an objective lens 1, and coils 4 for performing tracking and focusing are positioned at the outside of the magnets 3. A tracking control current and a focusing control current are made to flow into the coils 4 to drive the movable portion 2 of the actuator in the focusing direction (in the direction of the optical axis of the objective lens), in the tracking direction (in the direction perpendicular to the optical axis of the objective lens), etc. by forces generated between the coils 4 and the magnets 3.
In one example of a disk clamping apparatus for clamping a disk onto a turntable, a magnet is attached to the turntable, and a magnet plate such as an iron plate or the like is attached to a clamper for pressing a disk against the turntable. In another example, the magnet plate is attached to the turntable and the magnet is attached to the clamper. Such a disk clamping apparatus is arranged to clamp a disk between the clamper and the turntable by the force of attraction between the magnet and the magnetic plate, the clamper rotating as the disk rotates.
Generally, the magnet 5 used in the disk clamping apparatus is two-pole magnetized (hereinafter, the term "two-pole magnetization" means that there are two magnetized poles in both of the front and rear surfaces of the magnet) as shown in FIG. 8(b), so that a magnetic field is generated toward the magnetic plate to attract it to the magnet through a disk. As a result, even after the magnetic plate has been attracted to the magnet, a leakage field is generated from the magnet 5 in the form of an alternating field, because of the rotation of the clamper.
If an MM-type optical pickup is used in a optical disk player using such a disk clamping apparatus as described above, at the time of reproducing recorded information located at an inner circumferential portion of a disk, the magnet 3 attached on the movable portion 2 of the actuator comes closer to the clamper and to the turntable disposed at the central portion of the disk, so that the actuator movable portion 2 is affected by attractive/ repulsive forces due to the alternating field of the magnet 5 of the disk clamping apparatus (FIG. 8(a)). Such an influence appears as a force acting on the actuator movable portion 2 mainly in the tracking direction.
Moreover, the vibrations of the movable portion 2 of the actuator generated by the alternating field are made larger because of the frequency of the alternating field. That is, the rotating speed of a spindle motor or the frequency of the alternating field generated by the magnet 5 of the turntable, and the resonance frequency of the movable portion 2 of the actuator are of similar value. As a result, an error rate of a tracking error or the like is increased, reducing the reproducing capability of the optical disk player, or making the optical disk player reproduce so defectively as to be out of range of operation of a tracking servo, so that the servo cannot operate.
The resonance of the actuator movable portion now will be discussed in greater detail. When reproducing a compact disk, for example, the linear velocity V of the disk is determined to be 1.2-1.4 m/sec. Program index information (TOC) is recorded within a range of from r=23 mm to r=25 mm, where r is a radius from the center of the disk. It is known experimentally that the position where the influence of attraction/repulsion due to the magnet 5 of the turntable on the actuator movable portion 2 is substantially negligible is a position where r=28.5 mm. Therefore, the actuator position range of interest is from r=23.0 mm to r=28.5 mm.
The rotating speed of the motor (the magnet 5 attached to the turntable) in the foregoing range when the linear velocity of the disk is worst is as follows:
f=1400/2.pi.r=9.7 Hz when r=23.0 mm PA1 f=1200/2.pi.r=6.7 Hz when r=28.5 mm
Therefore, the rotating frequency (the frequency of the alternating field) of the magnet 5 ranges from 6.7 Hz to 9.7 Hz.
Generally, the magnet has a magnetization pattern in the form of a substantially rectangular wave, that is, the magnet is magnetized by rectangular wave magnetization. Therefore, if the magnet is rotated, frequency components other than a fundamental frequency are generated. Accordingly, a rectangular wave f(t) may be expressed as follows: EQU f(t)=4k/.pi.(sin t+1/3.multidot.sin 3t+1/5.multidot.sin 5t+. . . .) (1)
FIG. 4 shows the spectra of the third and fifth harmonics of the fundamental wave having a frequency of 7.5 Hz. In the drawing, k represents the degree of amplitude. Thus, the third, fifth, . . . . harmonics are generated in addition to the fundamental sine wave, but the level of those higher odd order harmonics become smaller as the order becomes higher. Thus, for example, the respective levels of the third and fifth harmonics are 1/3 and 1/5 as large as that of the fundamental sine wave. Only the third harmonic affects the magnetic field in addition to the fundamental wave; the influence of the fifth and higher order harmonics are substantially negligible in practical use.
Then, considering the magnet 5 (two-pole magnetization) of the turntable, it is understood that an additional region of concern exists in the frequency region of from f=20.1 Hz to f=29.1 Hz of the third harmonic in addition to in the frequency region of from f=6.7 Hz to f=9.7 Hz of the fundamental wave (FIG. 5(a)).
Therefore, there has been a problem in that if the low band resonance frequency f.sub.0 of the actuator coincides with frequency of attraction/repulsion by the magnet 5, the vibrations of the movable portion of the actuator are amplified so that reduction in reproducing capability of defective reproduction is caused in the optical disk player as described above.